My brother in-law and brew partner is gonna have a kid. I mentioned to him about a country that would age a beer until the kids 21st birthday and we both got really excited about this. We plan on making a 15% barly wine.

I haven't, (yet) but it sounds like a helluva idea, especially for a child to reach 21. I have heard of meads that old, and currently have one mead that I hope has a few bottles last that long, we shall see. Good luck!

Your biggest concerns are probably going to be (initially) attenuation/alcohol tolerance of your yeast, and then oxidation over 21 years- if you can get to 15% with all malt (consider brewing on a 10g system and boiling down to 5g) you'll be a rare brewer. I'd like to humbly suggest that you go with high attenuating, high alcohol tolerant yeast that is NOT WLP099 for the best results. Mash at a moderate temperature (152-153) for a LONG LONG time for best attenuation.

I would also bottle with oxygen-absorbing caps, in bottles with slightly less heaspace than usual, and wax the caps to prevent as much oxidation as possible.

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I can't be arsed to keep up this list of what's in the fermenters, but hey, check out the cool brewery I own!

You're probably gonna want to cork it like a champagne bottle, or dip the crown cap in a couple layers of sealing wax. Just about every commercial "aging beer" in my cellar comes one of those two ways, because just a naked crown cap won't keep the oxygen out over a multi-year timespan.

I imagine champagne-style corking is out-of-scope, but I seem to recall stumbling across a couple threads here on wax sealing...

I'd also not bother much with hops or spices -- I've had hoppy or spiced beers around five, seven years old, there wasn't much left at that age, I can't imagine there'd be anything there after two decades.

I'm really excited to do this. We're gonna take some pictures of us making it and probably throw a couple other things in. Thanks for the info on oxygen absorbing caps and waxing the top. That is a must! I'm thinking about finding some sort of box that I can put it in and putting some of those Silica bags in it that your not supposed to eat, or some other type of moisture absorbing material.

I've done 15+ beers before with a ton (1 straight package and one in a 1.8L starter on a stir-plate) of 1056 and everything turned out great.

I only have a 10 gal Igloo for a mash-tun so, I'm probably gonna max out the mash-tun and then do a BIAB side by side. I'll keep some DME on hand just in case I don't make it.

Brewing is an art and a science. I want to plan this out as much as possible but there will be a time that I just let go and brew and see what happens.